A TOP security jail has been placed on high alert over a birthday escape plot by Charles Bronson – dubbed Britain’s most dangerous convict.

He is 58 today. But after 36 years behind bars, 31 of them in solitary, a recent search by officers at Wakefield prison found that the window in his cell was unsecured.

A special staff meeting at the West Yorkshire facility – nicknamed Monster Mansion because it holds so many killers – heard the opening could lead him to an exercise yard.

And though superfit Bronson, who does 2,500 press-ups a day, would need to scale a wall to get his first taste of freedom since 1974, an insider said: “We know the significance of the window being disconnected.

“He remains very dangerous despite his age and could easily take out the watch in the yard before getting over the wall.”

Warders were also placed on alert over a possible kidnap attempt three weeks ago when he took on 12 of them in the gym after covering himself with butter.

In 10 jail sieges, he has taken a string of hostages, attacking 32 officers and causing £500,000 damage in rooftop protests.

First jailed for armed robbery in 1969, he has had only 69 days of freedom since.

He has been moved 121 times due to his appalling record, including spells in Broadmoor and Rampton. And the insider said: “There are concerns about Charlie’s state of mind. He is again finding it difficult to get his head around being locked up for life.

“He’s been co-operative during searches but his behaviour has changed since the gym incident. He’s a bomb waiting to go off.”

Born Michael Peterson, Bronson changed his name to honour the late Death Wish film star when he was a bare-knuckle fighter.

Last jailed in 1974, he kidnapped prison teacher Phil Danielson inside Hull jail in 1999 and was told he must serve life. An appeal failed last year.

He has been the subject of a movie and his artwork can earn him £2,500 a canvas. Now he refuses to see prison psychiatrists.